1. Technical Field of the Invention
The present invention relates to a scanning optical system of laser beam printer and more particularly to a scanning optical system which is arranged for irradiating a laser beam to such rotary optical deflectors as a rotating polygon mirror and a Galvano mirror which rotates or sways centering on an axis and repeatedly and successively deflecting the beam within a constant angle thereby scanning the surface of a recording medium for forming an image thereon.
2. Brief Description of the Prior Art
In a scanning optical system of a laser beam printer which adopts such methods for image formation as described above, a laser beam which is deflected by a rotary optical deflector waves centering on almost a constant position of a point of deflection. As a result, the velocity of laser beam for scanning the surface of a recording medium straight to the scanning direction becomes faster at both end portions of the recording media and slower in the central portion.
U.S. Pat. No. 4,251,821 discloses a laser beam printer which makes the diameter of a dot to be used as an element of recording picture to be constant without having any influence by the variation of velocity in the scanning direction. The device is arranged to emit light for a longer time per dot when the scanning point of a laser beam approaches the periphery of a recording medium. Accordingly, the size of the dot becomes uniform in the scanning direction. However, the light amount to be given by the laser beam to the periphery of the recording medium and to the central portion can not be equal due to the difference in scanning velocity. In other words, the light amount which is received at the periphery of a recording medium wherein the scanning velocity is faster is reduced per unit area, and at the central portion where the scanning velocity is conversely slower is increased. Accordingly, the exposure state on the recording medium in the scanning direction is changed on the same image density and is caused to lower the quality of image to be formed.